In synchronous time-division multiplex digital data transmission systems, the transmission of data from a transmitter terminal to a receiver terminal requires framing or timing information to properly demultiplex the received data. The transmitter terminal typically generates bit and framing pulses to align the outgoing bits in appropriate time slots, assembles these bits into frames having a fixed number of bits, and separates adjacent frames with framing bits which define the boundaries between successive frames. The receiving terminal detects the framing signal, disassembles the incoming frames and recovers the bits in each of the time slots.
When noise or other interfering signals in the transmission paths cause errors in the data stream, a loss of framing may occur causing the receiver to reframe. Reframing involves searching through all the bit positions of the received data stream looking for the correct framing sequence.
Prior art framing methods basically utilize either added digit framing, robbed digit framing or statistical framing using an intrinsic characteristic of the data signal. Added digit framing requires the addition of framing bits to the data stream thus reducing the overall information transmission rate. In robbed digit framing certain signal data bits are replaced by framing bits resulting in some signal impairment. In statistical framing the intrinsic data signal requires no dedicated digits but is not as reliable as the other framing methods.
Thus, a continuing problem exists to develope a digital framing method which minimizes the deleterious effects of the framing bits on the information transmission rate, accuracy of the information signal, or reliability of achieving framing. Additionally, it is a problem to minimize the circuitry required to implement the framing method utilized.